The present invention relates to a liquid crystal display element free from interference colors and whose contrast has a very small angular dependence.
The characteristics of nematic or nematic-cholesteric liquid-crystalline materials whereby they significantly vary their optical properties, such as light transparency, light scattering, birefringence, reflectivity or color, under the influence of electric fields are, utilized for liquid crystal display elements. The functioning of such display elements is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases or the Schadt-Helfrich effect in the twisted cell.
Among these conventional types of liquid crystal display elements, especially those based on the twisted nematic cell have recently gained particular importance, since they can be operated at relatively low control voltages. These can be readily provided even by small batteries. Moreover, hitherto these display elements have proven to be the most suitable for the construciton of matrix display elements. These permit presentations of high information density without an intolerably large number of triggering leads, input leads and output leads.
In practice, however, considerable difficulties still persist in using the twisted nematic cell, especially when in the form of matrix display elements. In particular, a marked dependence of the display contrast on the angle of observation is regularly found. Display contrast is good as long as the direction of observation is at least approximately perpendicular to the plane of the liquid crystal layer. If, however, the display element is observed obliquely from the side, for example, at angles of observation of more than 15 to 20 degrees from the perpendicular, the display contrast decreases strongly until finally--depending on the position of the polarizer facing the observer--the display is no longer perceptible. Additionally, interference colors frequently occur. That is, black-and-white display elements additionally show iridescent colors which, depending on irregularities in the layer of the electrode surface which faces the liquid crystal dielectric, cause a display of colors over the entire spectrum of visible light.
From work by Mauguin (Bull. Soc. Franc. Min., Volume 34, 1911, pages 71-117) relating to the behavior of liquid crystals between polarizers, it has been deduced that at least the interference phenomena can be prevented in the liquid crystal cell, if the product of the layer thickness and the optical anisotropy of the liquid crystal material is substantially greater than the wavelength of the light used. The optical anisotropy of a liquid crystal material is defined as the difference between the extraordinary index of refraction and the ordinary index of refraction. In practice, it is accepted as a rule of thumb that the value of the product of the layer thickness and the optical anisotropy must not be less than 1,400 nm; the product specifications of known electronics manufacturers prescribe values of more than 2,000 nm for this product. Raising this value by increasing the layer thickness of the liquid-crystalline dielectric is, however, limited by the fact that the switching time of the liquid crystal material increases with the square of the layer thickness. With the nowadays customary layer thicknesses of 10 to 12 .mu.m, liquid-crystalline dielectrics are therefore demanded, which have an optical anisotropy of at least 0.14 and preferably more than 0.18.
Although in this way the difficulties due to the generation of interference colors can largely be eliminated in the construction and use of the twisted nematic cell, the strong angular dependence of the constrast is hardly improved thereby. Moreover, the fact that any reduction of the layer thickness potentially causes a disturbance due to interference colors, has precluded such adjustments in developing liquid crystal display elements which switch more rapidly, such as are required, for example, for use as a television screen.